The present invention relates to a highly oxidation-resistant, mechanically high-strength carbonaceous material or, more particularly, to a carbonaceous sintered body mainly composed of a coke powder and having a greatly improved oxidation resistance and high mechanical strengths in comparison with conventional carbonaceous materials.
As is well known, carbonaceous materials are imparted with various excellent properties such as high mechanical strengths at high temperatures, resistance against heat shock, high thermal conductivity, good electric conductivity, low thermal expansion, outstanding chemical stability and others. Carbonaceous materials, however, are not free from a very serious disadvantage that they are highly susceptible to oxidation when heated at a high temperature in an oxidizing atmosphere. For example, oxidation of a carbonaceous body heated in air at a gradually increasing temperature begins at about 300.degree. C. and rapidly proceeds when the temperature exceeds 600.degree. C. Therefore, carbonaceous materials can be used at high temperatures only in a non-oxidizing atmosphere and cannot be used in the atmospheric air despite their excellent high temperature performance in general greatly limiting the fields of their industrial applications.
Of course, there have been many attempts to improve the oxidation resistance of carbonaceous materials including a method in which a carbonaceous material is impregnated with a phosphate compound, a method in which a carbonaceous material is impregnated or coated with a specific component capable of forming a vitreous surface film and a method in which a carbonaceous material or graphite before shaping is admixed with silicon carbide and a ferrosilicon or a combination of silicon carbide and a siliceous binder mainly composed of silicic acid. None of these prior art methods is, however, quite satisfactory. For example, the method of impregnation with a phosphate compound is no more than a means to increase the beginning temperature of the oxidation of carbonaceous materials only by 100.degree. to 200.degree. C. and no good solution of the problem is provided thereby. The second method of impregnation or coating with a vitreous film-forming component is, although effective to some extent, not able to give a stable effect of increasing the oxidation resistance of a carbonaceous material since the principle of the method depends solely on the protection provided by the vitreous film formed on the surface of the carbonaceous material which is not free from unevenness or damage as well as peeling of the film off the surface due to the difference between the coefficients of thermal expansion of the carbonaceous material and the vitreous surface film eventually to cause localized oxidation of the carbonaceous material. The third method of formulating a carbonaceous material or graphite with an additive including silicon carbide and a binder is considerably effective for the improvement of the oxidation resistance thereof but the effectiveness of the method is far from satisfactory. For example, such a carbonaceous material may lose 10 to 30% of the weight when it is heated in air at 900.degree. C. for 5 hours. Moreover, the addition of such a binder component adversely affects the densification of the sintered body necessarily leading to a great decrease in the mechanical strengths of the sintered body.
On the other hand, some of the inventors have previously proposed (see Japanese Patent Kokai No. 56-140075) a method for the preparation of a carbon-ceramic composite material by sintering a compression-molded body of a powdery mixture of a pulverized specific coke and a ceramic powder such as silicon carbide SiC, boron carbide B.sub.4 C or alumina Al.sub.2 O.sub.3 in a relatively large proportion of, for example, 30 to 50% by weight in the mixture. The carbon-ceramic composite body prepared by this method is, however, not satisfactorily resistant against oxidation at high temperatures and loses 5% or more of weight when heated in air at 1000.degree. C. for 5 hours.
Thus, no method is known in the prior art suitable for the preparation of a carbonaceous material highly resistant against oxidation in air when heated at 1000.degree. C. or higher.